C150 MAKING TOUCH AND GO LNDGS HAS NMAC WITH TOUR HELI.

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MAKING APCH TO UNCTLED FIELD. CALL IN TO MULTICOM AT 1500 FT; 1200 FT XING MID-FIELD; 800 FT ON DOWNWIND FOR RWY 8 FOR 'TOUCH AND GOES.' 3 TOUR HELIS ARE PARKED ON THE GRASS BESIDE THE RAMP. THE APCH END OF FIELD IS OBSCURED BY BUSHES AND TREES. AS I FLARED FOR LNDG (OVER THE NUMBERS) A HELI CAME OUT FROM THE GRASS OVER THE RWY INTO (OR SLIGHTLY ABOVE) MY PATH; TURNED 180 DEGS IMMEDIATELY BACK OFF THE RWY TO AVOID ME AND THEN DEPARTED. I CONTINUED ON WITHOUT LNDG TO KEEP FLYING SPD IN ANTICIPATION OF BLAST OF WIND FROM HIS ROTOR BLADES; AND I REENTERED THE PATTERN AND MADE A NORMAL FULL STOP LNDG. CONTRIBUTING FACTORS: I FAILED TO ANNOUNCE BASE AND FINAL TURNS WHICH MAY HAVE STOPPED HIM FROM LIFTING OFF; BUSHES OBSCURE THE APCH END OF RWY 8 FROM THE HELI LNDG AREA; HELIS USE THAT AREA AS A LOADING AND UNLOADING AREA FOR TOUR PAX -- COMMERCIAL OP; AND HELIS USE SOME DIFFERENT FREQS IN THAT AREA AND ACFT #2 MIGHT NOT HAVE BEEN MONITORING 122.9 MULTICOM (ALTHOUGH ONE OF THE PLTS ACKNOWLEDGED MY APCH AND SAID HE'D BE LOOKING OUT FOR ME).